1. Technical Field
The present invention relates to a substrate before an insulation process, which is provided with a protection film to prevent a part of an surface area, which has electrical conductivity, from being treated with insulation, a method of manufacturing a substrate with being insulated, a method of manufacturing a surface acoustic wave transducer, a surface acoustic wave transducer manufactured by this method, a surface acoustic wave device, and electronic equipment.
2. Related Arts
A surface acoustic wave transducer for electrically driving surface acoustic wave (SAW) or detecting it is manufactured by forming an interdigital transducer (IDT), a reflector and a conductive pad made of an aluminum thin film on the surface of a piezoelectric substance such as a quartz. In such surface acoustic wave transducer, attaching a conductive foreign materials to the surface of an IDT causes a problem of fluctuation of frequency and instability of resonance characteristics. In order to avoid the issue, it is preferable that the surface of an IDT is covered by an insulating material. On the other hand, it is preferable that the surface of a conductive pad maintains its conductivity without being covered by an insulating material since the pad is a portion for connecting an outer wiring.
JA11-330882 discloses a technology for manufacturing a surface acoustic wave transducer satisfying this requirement. In this technology, a resist film is formed as a protection film against anodic oxidization on a conductive pad by photolithography and an oxide film is formed only on the surfaces of a IDT and a reflector by the anodic oxidization. Then, after removing the resist film, an aluminum thin film on the surface of a conductive pad is exposed while the surfaces of a IDT and a reflector are covered by an oxide layer.
Recently, a method is suggested wherein a resist as a protection film is selectively given only to a conductive pad by an inkjet method. According to this method, an amount of a used resist can be reduced. Further, no mask is needed because of omitting a process for developing a resist, constraining a manufacturing cost. It also prevents the surface of an aluminum thin film from deterioration due to a resist development liquid.
However, if a resist is given by an inkjet method, the thickness of a resist is insufficient. This insufficiency of the thickness of a resist at the circumferential area, in particular, causes an anodic oxidation liquid to leak in the interface between a conductive pad and a resist film from the circumference of a resist film during anodic oxidation process, forming an oxide film in a part of a region in which a resist film is placed. Further, the insufficient thickness at the center of a resist film causes an insufficient strength against an anodic oxidation liquid at this part (the strength of adhesiveness), making a part of the center area of a resist film remove during a anodic oxidation process. In this case, an oxide film is formed in a part of a region in which an oxide film should not be formed.